The effect of TEMPO in the hydroxylation of benzene to phenol on the [(CH3)4N]4PMo11VO 40/ascorbic acid/TEMPO/O2 catalytic system: Formation of ascorbic acid radicals through hydrogen excha

Article abstract of DOI:10.1016/j.apcata.2011.11.032

Hydroxylation of benzene to phenol in the [(CH₃) ₄N]₄PMo₁₁VO₄₀/ascorbic acid/TEMPO/O₂ catalytic system was carefully investigated. UV-vis, ESR and ¹H NMR studies showed ascorbic

Full text of DOI:10.1016/j.apcata.2011.11.032

Applied Catalysis A: General 415–416 (2012) 22–28
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
The effect of TEMPO in the hydroxylation of benzene to phenol on the
[(CH3)4N]4PMo11VO40/ascorbic acid/TEMPO/O2 catalytic system: Formation of
ascorbic acid radicals through hydrogen exchange of ascorbic acid and TEMPO
Hua Yanga,b,c, Jia-Qi Chena,b,c, Jun Lia,b, Ying Lva,b, Shuang Gaoa,b,∗
a Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China
b Dalian National Laboratory for Clean Energy, Dalian 116023, People’s Republic of China
c Graduate School of the Chinese Academy of Sciences, Beijing 100039, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Hydroxylation of benzene to phenol in the [(CH3)4N]4PMo11VO40/ascorbic acid/TEMPO/O2 catalytic sys-
tem was carefully investigated. UV–vis, ESR and 1H NMR studies showed ascorbic acid radicals were
formed through the hydrogen exchange of ascorbic acid with TEMPO (2,2,6,6-tetramethyl-1-piperidine-
N-oxyl radicals) during the reaction. In acetonitrile, the ascorbic acid would be partly oxidized to
2-hydroxy-2-buten-4-olide (␣-tetronic acid, isotetronic acid, compound 1) without TEMPO. The interac-
tion of TEMPO and ascorbic acid restrained the oxidative dissociation of ascorbic acid and promoted the
rate of the hydroxylation. Aqueous acetic acid solvent can also restrain the oxidative dissociation of ascor-
bic acid. In aqueous acetic acid, the yield of phenol could reach 18.9% in the [(CH3)4N]4PMo11VO40/ascorbic
acid/TEMPO/O2 catalytic system with sufficient ascorbic acid after 400 min.
Received 25 October 2011
Received in revised form
28 November 2011
Accepted 29 November 2011
Available online 8 December 2011
Keywords:
TEMPO
Ascorbic acid
Oxygen
© 2011 Elsevier B.V. All rights reserved.
Benzene
Phenol
1. Introduction
phenol yield was 0.23%. In 1995 Tsuruya [12] used zeolites loaded
with Cu as a catalyst, with a phenol yield of 1.69%. V-based catalysts
Phenol is an important chemical material in producing phenol
resins, bisphenol-A, dyes, antioxidation agents and pharmaceu-
ticals [1–5]. The traditional method is the cumene process [6]
which accounts for above 90% phenol production in the world. The
cumene process is restricted by the co-product acetone, and causes
a lot of environmental problems. Many new methods for hydroxy-
lation of benzene to phenol have been developed in recent decades
[7,8]. Among them the direct hydroxylation of benzene to phenol by
molecular oxygen is preferred. In 1954, Udenfriend and co-workers
first suggested an ideal oxidation system in which oxygen could be
used as the oxidant in the oxidation of aromatic hydrocarbon [9,10].
[22–26] cooperating with ascorbic acid had been widely investi-
gated and achieved good phenol yield. Tsuruya and co-workers
[24] used CsPMoV2 as the catalyst, aqueous acetic acid as the sol-
vent, and the yield of phenol reached 7.2% after 1440 min. Later,
Wang et al. [26] used Py3PMo10V2O40 as catalyst, aqueous acetic
acid as the solvent, and the yield of phenol reached 11.5% after
600 min. However, most of the catalytic systems involving oxygen
and ascorbic acid have a low rate of hydroxylation.
In our previous work [27], it was discovered that
TEMPO can improve the rate of the hydroxylation in
[(CH3)4N]4PMo11VO40/ascorbic acid/O2 catalytic system, but the
real role of TEMPO was not very clear. In the present work, hydrox-
ylation of benzene to phenol in the [(CH3)4N]4PMo11VO40/ascorbic
acid/TEMPO/O2 catalytic system was carefully investigated.
In this process, ascorbic acid was used as the co-reductant, Fe2+
-
EDTA as the catalyst, and the reaction vessel charged with oxygen
as the oxidant. Since that time, various types of catalytic hydrox-
ylation processes containing Cu [11,12], Pd [13–17], Pt [18,19] or
Re [20,21] have appeared in the literature. In 1987, Orita [11] used
CuCl as the catalyst and ascorbic acid as the co-reductant, and the
2. Experimental
2.1. Catalyst preparation
H4PMo11VO40 was prepared according to reference with minor
modifications [28]. 7.16 g (20 mmol) of Na2HPO4·12H2O and 3.16 g
(20 mmol) of NaVO3·2H2O were dissolved in 80 mL of water at
room temperature. 2 mL of concentrated H2SO4 was added and
∗
Corresponding author at: Dalian Institute of Chemical Physics, Chinese Academy
of Sciences, Dalian 116023, People’s Republic of China. Tel.: +86 0411 84379248;
fax: +86 0411 84379248.
E-mail address: sgao@dicp.ac.cn (S. Gao).
0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.11.032

H. Yang et al. / Applied Catalysis A: General 415–416 (2012) 22–28
23
the mixture was acidified to a red color, 53.2 g (220 mmol) of
Na2MoO4·2H2O was dissolved in 80 mL of water, then added to
this mixture. 34 mL of concentrated H2SO4 was added to the solu-
tion. After vigorous stirring of the mixture for 1 h, 160 mL of ethyl
ether was added to extract the heteropoly acid. H4PMo11VO40
was identified by IR spectra (see in Fig. S1). [(CH3)4N]4PMoV11O40
was prepared by adding H4PMo11VO40 into the solution of four
equivalents of (CH3)4NOH and characterized by IR (Fig. S1), ele-
mental analysis (C/H/N was 4.05/11.76/1.00), ICP (P/Mo/V was
1.00/11.2/1.01), 31P MAS NMR (Fig. S2), and XRD (Fig. S3) [29].
2.2. The hydroxylation of benzene
The hydroxylation of benzene was carried out in a stainless
steel reactor equipped with a 40 mL PTFE liner. In a typical reac-
tion, 0.105 g (0.05 mmol) of catalyst [(CH3)4N]4PMo11VO40, 0.78 g
(10 mmol) of benzene, 0.156 g (1 mmol) of TEMPO, 0.9 g (5 mmol)
of ascorbic acid were added into 6.8 mL of acetonitrile or 50% aque-
ous acetic acid. The reactor was pressurized with O2 to 2.0 MPa and
put into a water bath at 80 ◦C for 80 min. A magnetic stirrer was
used during the process. After reaction, 1,4-dioxane was added as
an internal standard.
Fig. 1. Oxygen uptake in different reaction conditions. Reaction conditions: Ascor-
bic acid 0.9 g (5 mmol), benzene 0.78 g (10 mmol), acetonitrile 6.8 mL, 80 ◦C,
initial pressure of 0.8 MPa O2. (a) No [(CH3)4N]4PMo11VO40 and TEMPO, (b)
[(CH3)4N]4PMo11VO40 0.105 g (0.05 mmol), (c) TEMPO 0.156 g (1 mmol), and (d)
[(CH3)4N]4PMo11VO40 0.105 g (0.05 mmol), TEMPO 0.156 g (1 mmol).
2.3. Materials and characterization
indicated that TEMPO accelerated the uptake of oxygen in the reac-
tion. It was reported that ascorbate could react quickly with TEMPO
[30,31] with the loss of a proton and an electron [32], generating
ascorbic acid radicals and TEMPOH. We supposed that the reac-
tion between ascorbic acid and TEMPO is a similar process (proved
in Section 3.2). It was probable that the drastic increase of oxy-
gen was caused by the reaction between ascorbic acid radicals and
oxygen, and the reoxidation of TEMPOH. We used GC to follow the
interaction of TEMPO, ascorbic acid and oxygen: after TEMPO and
ascorbic acid (1:5 molar ratio) was mixed, TEMPO was detected to
form TEMPOH by GC. After the mixture was charged with oxygen,
TEMPOH could change back to TEMPO. TEMPO and TEMPOH were
detected to coexist in the mixture by GC. The mechanism involved
in ascorbic acid and TEMPO was proposed as shown in Scheme 1.
The oxygen uptake was partly promoted by the interaction
between ascorbic acid and TEMPO (Fig. 1, curve c) and was fur-
ther promoted by the catalyst together with TEMPO (Fig. 1, curve
d). Therefore, the most drastic oxygen uptake must be caused by
Na2HPO4·12H2O (99%, Tianjin Kermel Chemical Reagent Co.,
Ltd.), Na2MoO4·2H2O (99%, Tianjin Kermel Chemical Reagent Co.,
Ltd.), NaVO3·2H2O (98%, Tianjin Kermel Chemical Reagent Co., Ltd.),
(CH3)4NOH (97%, Aladdin Chemistry Co., Ltd.), benzene (99.5%, Bei-
jing Chemical Works), acetic acid (99.5%, Guangdong Guanghua
Chemical Factory Co., Ltd.), acetonitrile (99.5%, Beijing Chemical
Works), ascorbic acid (99.7%, Tianjin Kermel Chemical Reagent Co.,
Ltd.) and TEMPO (98%, Aladdin Chemistry Co., Ltd.) were of analyt-
ical grade and were used as received without further purification.
The GC of the samples was analyzed by a GC-Agilent 7890 series
instrument equipped with a capillary column (Agilent, SE-30) and
a flame ionization detector (FID).
The GC–MS of the samples was analyzed by a Shimadzu-15 A
GC–MS equipped with a capillary column (GL Sciences, TC-FFAP)
and a flame ionization detector (FID).
The UV–vis spectra of the samples were recorded on
a
Shimadzu-2550 UV–vis spectrophotometer. Quartz sample cells
were of 10-mm path length. The backgrounds of UV–vis spectra
were subtracted using only solvent (acetonitrile) as a reference
sample. The samples were recorded at ambient temperature.
The ESR spectra of the samples were recorded on a JEOL ES-ED3X
spectrometer at X-band at ambient temperature.
The 1H NMR spectra of each sample were recorded on a Bruker
DPX 400 MHz type spectrometer at ambient temperature in DMSO-
d6 using TMS as an internal standard with reference shift at 0 ppm.
3. Results and discussion
3.1. Oxygen uptake in different reaction conditions
The oxygen uptakes under different reaction conditions were
firstly investigated (Fig. 1). Without catalyst and TEMPO, the oxygen
pressure variation was almost negligible (Fig. 1, curve a), showing
that the reaction between ascorbic acid and oxygen was slow.
With the addition of catalyst, it can be seen that the oxygen
uptake was slightly increased in the initial 40 min (Fig. 1, curve b). It
was confirmed that catalyst slightly accelerated the oxygen uptake
in the reaction in the initial 40 min. Beyond 40 min, acceleration by
the catalyst was more obvious.
After TEMPO was added in the reaction (Fig. 1, curve c), there
was a drastic increase in oxygen uptake in the first 10 min. This
Scheme 1. Mechanism for the reaction between TEMPO and ascorbic acid in ace-
tonitrile.

24
H. Yang et al. / Applied Catalysis A: General 415–416 (2012) 22–28
Fig. 2. UV–vis spectrum of (a) ascorbic acid, (b) ascorbic acid mixed with TEMPO and
(c) TEMPO. Reaction conditions: (a) 2.5 mM ascorbic acid in acetonitrile, (b) 0.5 mM
TEMPO, 2.5 mM ascorbic acid in acetonitrile, and (c) 0.5 mM TEMPO in acetonitrile.
the combined catalytic system involving ascorbic acid, catalyst and
TEMPO.
3.2. Identification of ascorbic acid radicals by UV–vis, ESR and 1
NMR
H
To elucidate the nature of the interaction between ascorbic acid
and TEMPO, the mixture of ascorbic acid and TEMPO was measured
by UV–vis (Fig. 2) and followed over 80 min (Fig. 3). The maxi-
mum absorption band around 238 nm was designated as ascorbic
acid (Fig. 2, curve a). The maximum absorption band of TEMPO
was 240 nm (Fig. 2, curve c). The UV–vis spectrum of ascorbic acid
and TEMPO were constant within 80 min. However, when ascorbic
acid and TEMPO were mixed together, a new absorption band was
quickly generated (Fig. 2, curve b).
The maximum absorption band of 236 nm decreased with the
increase of reaction time (Fig. 3). This band was suggested to be
the absorption of ascorbic acid radicals. Because of the decaying
of ascorbic acid radicals, this signal slowly decreased during the
80 min.
Fig. 4. 1H NMR of ascorbic acid, the mixture of ascorbic acid and TEMPOH DMSO-
d6 as the solvent. Reaction conditions: (a) 0.049 g ascorbic acid dissolved in 0.5 mL
DMSO-d6 and (b) 0.049 g ascorbic acid and 0.065 g TEMPO dissolved in 0.5 mL
DMSO-d6.
To further illustrate the interaction between ascorbic acid and
TEMPO, ascorbic acid (Fig. 4a) and the mixture of TEMPO and ascor-
bic acid (Fig. 4b) were analyzed by 1H NMR. Ascorbic acid was stable
at ambient temperature, and its 1H NMR is shown in Fig. 4. After the
mixing of ascorbic acid and TEMPO, the 1H NMR signals of ascorbic
acid disappeared from the 1H NMR spectrum. Instead, two new sig-
nals caused by the formation of TEMPOH appeared in the 1H NMR
spectrum. The ratio of the chemical shift at around 1.4–1.0 ppm was
6:12, and designated as the hydrogen of methylene and methyl of
TEMPOH.
To identify the new species, the interaction of ascorbic acid
and TEMPO was also followed by ESR. The sample of TEMPO in
acetonitrile gave a strong ESR signal (g = 2.0055) at ambient tem-
perature as shown in Fig. 5a. The same amounts of ascorbic acid and
TEMPO were added to the solvent above and the ESR signals were
transformed to the new signal (Fig. 5b). This transformation of ESR
spectroscopy was caused by the formation of another species with
an unpaired electron. The signal was centered at g = 2.0059, which
was the same as the report of ascorbic acid radicals in acetonitrile
[31] and confirmed our assignment.
3.3. Tracking analysis of the hydroxylation process by GC in situ
When the hydroxylation of benzene to phenol was carried out
without TEMPO, a new compound in addition to phenol was found
in acetonitrile (Fig. 6). This compound was 2-hydroxy-2-buten-
4-olide (␣-tetronic acid, isotetronic acid), which was determined
by GC–MS (Fig. S4) and named as compound 1. According to the
structure of compound 1, it was regarded as a product of oxida-
tive dissociation of ascorbic acid. The phenomenon of the oxidative
Fig. 3. UV–vis absorption decaying of the maximum of the mixture of TEMPO and
ascorbic acid. Reaction conditions: 0.5 mM TEMPO reacted with 2.5 mM ascorbic acid
in acetonitrile over 80 min.

H. Yang et al. / Applied Catalysis A: General 415–416 (2012) 22–28
25
Fig. 5. ESR spectroscopy of TEMPO and ascorbic acid radicals. Reaction conditions: (a) 0.5 M TEMPO, acetonitrile as the solvent and (b) 0.5 M ascorbic acid, 0.5 M TEMPO,
acetonitrile as the solvent.
Fig. 6. Hydroxylation of benzene to phenol without the addition of TEMPO (red line indicated phenol, green line indicated compound 1). Reaction conditions:
[(CH3)4N]4PMo11VO40 0.105 g (0.05 mmol), ascorbic acid 0.9 g (5 mmol), benzene 0.78 g (10 mmol), acetonitrile 6.8 mL, 80 ◦C, 2 MPa O2. (a) Compound 1 (retention time
is 8.7 min) and (b) phenol (retention time is 9.1 min). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the
article.)

26
H. Yang et al. / Applied Catalysis A: General 415–416 (2012) 22–28
Fig. 7. Compound 1 was depressed by the addition of TEMPO (red line indicated phenol, green line indicated compound 1). Reaction conditions: [(CH3)4N]4PMo11VO40 0.105 g
(0.05 mmol), TEMPO 0.156 g (1 mmol), ascorbic acid 0.9 g (5 mmol), benzene 0.78 g (10 mmol), acetonitrile 6.8 mL. 80 ◦C, 2 MPa O2. (a) Compound 1 (retention time is 8.7 min)
and (b) phenol (retention time is 9.1 min). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
dissociation of ascorbic acid has never been reported in the hydrox-
ylation of benzene. The low rate of hydroxylation is likely to be
caused by the oxidative dissociation of ascorbic acid that decreased
its efficiency in the hydroxylation (Table 1, right).
3.4. Effect of aqueous acetic acid on the hydroxylation reaction
Aqueous acetic acid was widely used as a solvent in the hydrox-
ylation of benzene to phenol [23–26]. Why is aqueous acetic acid
good for the hydroxylation of benzene? We found compound 1 can-
not be detected when aqueous acetic acid was used as the solvent.
That is to say, the oxidative dissociation of ascorbic acid did not
happen, meaning its efficiency was higher in aqueous acetic acid
than in acetonitrile. Table 2 shows that the yield of phenol was up
to 6.6% using aqueous acetic acid as the solvent. With the addi-
tion of TEMPO, there was a further increase of the phenol yield
from 6.60% to 9.00%. It was found that the interaction of ascorbic
acid and TEMPO was still happening in acetic acid/water. The extra
phenol yield may be attributed to the interaction between TEMPO
and ascorbic acid.
TEMPOH can be reoxidized to TEMPO by oxygen in acetoni-
trile with or without catalyst (Scheme 1). However, at acidic pH,
it was proposed by Neumann [33] that the reoxidation of TEM-
POH to TEMPO may be rate-determining, and it was very slow.
Sheldon [34] also reported that the nitrogen atom of TEMPOH
was protonated and less susceptible to oxidation. In our catalytic
After TEMPO was added into the reaction, the formation of com-
pound 1 was restrained (Fig. 7). Only traces of compound 1 can be
detected in the first 40 min, then it cannot be detected at all. It
was supposed that the oxidative dissociation of ascorbic acid was
restrained by the interaction of TEMPO and ascorbic acid (Fig. S5).
The oxidative dissociation of ascorbic acid decreased its efficiency
and was a negative factor for the hydroxylation process. There-
fore, the restraint of this oxidative dissociation of ascorbic acid
enhanced its efficiency and increased the yield of phenol (Table 1,
left).
To confirm that the oxidative dissociation of ascorbic acid was
restrained by the interaction of TEMPO and ascorbic acid, the con-
trolled experiments were carried out as shown in Fig. S5. Compared
with Fig. 6, only trace levels of compound 1 can be detected with the
addition of TEMPO. In acetonitrile, the mechanism for the hydrox-
ylation rate promoted by TEMPO in acetonitrile was proposed as
shown in Scheme 2.
Table 1
Effect of the reaction time on the phenol yield.
Reaction time (min)
Yield of phenol (%)a
Selectivity to phenol (%)a
Yield of phenol (%)b
Selectivity to phenol (%)b
10
20
40
60
80
120
240
360
3.76
4.25
5.12
5.95
6.96
7.25
7.35
7.40
>99
>99
>99
>99
>99
>99
>99
>99
2.51
2.66
2.97
3.17
3.48
3.87
4.55
5.26
>99
>99
>99
>99
>99
>99
>99
>99
Reaction conditions: Catalyst 0.105 g (0.05 mmol), ascorbic acid 0.9 g (5 mmol), benzene 0.78 g (10 mmol), acetonitrile 6.8 mL, 80 ◦C, 2 MPa O2.
a
With the addition of TEMPO 0.156 g (1 mmol).
Without TEMPO 0.156 g (1 mmol).
b

H. Yang et al. / Applied Catalysis A: General 415–416 (2012) 22–28
27
Scheme 2. Mechanism for the hydroxylation rate promoted by TEMPO in acetonitrile.
Table 2
Effect of the solvents on the phenol yield.
Yield of phenol (%)a
Selectivity to phenol (%)a
Yield of phenol (%)b
Selectivity to phenol (%)b
Acetonitrile
Aqueous acetic acid
6.96
9.00
>99
>99
3.48
6.60
>99
>99
Reaction conditions: Catalyst 0.105 g (0.05 mmol), ascorbic acid 0.9 g (5 mmol), benzene 0.78 g (10 mmol), acetonitrile or 50% aqueous acetic acid 6.8 mL, 80 ◦C, 2 MPa O2,
80 min.
a
With the addition of TEMPO 0.156 g (1 mmol).
Without TEMPO 0.156 g (1 mmol).
b
system, did the reoxidation of TEMPOH happen using aqueous
acetic acid as solvent? Some controlled experiments were car-
ried out to determine this. In aqueous acetic acid, the TEMPOH
generated from the interaction between TEMPO and ascorbic acid
could not be reoxidized to TEMPO without catalyst. After the
addition of catalyst, TEMPO can be detected. This showed that
the reoxidation of TEMPOH could happen with the assistance of
the catalyst in aqueous acetic acid (Scheme 3). To confirm the
recycling of TEMPO in the hydroxylation process, TEMPO as the co-
catalyst was added just once, ascorbic acid was added a further five
Scheme 3. Mechanism for the hydroxylation rate promoted by TEMPO in aqueous acetic acid.

28
H. Yang et al. / Applied Catalysis A: General 415–416 (2012) 22–28
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.apcata.2011.11.032.
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1. The oxidative dissociation of ascorbic acid happened in the
[(CH3)4N]4PMo11VO40/ascorbic acid/O2 catalytic system and
decreased ascorbic acid efficiency for the hydroxylation reaction
in acetonitrile. With TEMPO addition, the interaction of TEMPO
and ascorbic acid could restrain the oxidative dissociation of
ascorbic acid and increase the rate of the hydroxylation.
2. TEMPO can react quickly with ascorbic acid and generate ascor-
bic acid radicals and TEMPOH. TEMPO can be recycled in the
reaction through the reoxidation of TEMPOH. However, in aque-
ous acetic acid, catalytic assistance is needed for the recycling
process. Ascorbic acid radicals are more reactive than ascorbic
acid for the hydroxylation of benzene to phenol.
3. Aqueous acetic acid can protect ascorbic acid from oxidative dis-
sociation and TEMPO can also promote the rate of hydroxylation
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This work was supported by the National Basic Research Pro-
gram of China (973 Program, 2009CB623505).